A known method is to close the pores of selected areas of the thermo-plastic porous material by feeding it through heated rollers with engraved pattern(s) as in European Patent No. 0,553,808, or by pressing it with the heated face of a master mould, which has indentations to prevent it from heating and pressing the surface elements with the pores to be left open (GB patent No 1112776, B 44 C 1/24, Bankers and Merchants, Inc., 1968; JP patent No 6024114, B 41 K 1/50, Tokyo Ohka Kogyo Co.,Ltd., 1994). This method is used beside with other applications at the manufacturing of pre-inked stamps to selectively produce on the surface of porous thermo-plastic material a solid film, non-permeable for ink. The method is labour-consuming, as the manufacturing of each stamp of any new typesetting or picture requires the preparation of a new master mould.
Cross-linking and solidifying of molecules of polymer materials with light radiation has been widely used for forming not only printing surfaces but even complex three-dimensional parts of different shapes, but this method is known only for solid, nonporous materials (KONSTRUKTION, vol. 38, no. 9, September 1986, Berlin, pp. 347-48, E. V. Fudium: ("Sculpting Parts with Light")
Another known method is the manufacturing of printing surfaces from porous material with the help of laser radiation, where the surface of porous material in background areas is burnt and evaporated, thus creating the necessary relief (JP patent No 6191002, B 41 C 1/50, Mitsubishi Pencil Co. Ltd., Tsukasa Felt Shoji KK., 1994). A disadvantage of this method is that a sophisticated high-power laser is needed. Another problem is that at the scanning speeds usually accepted, provided the radiant power is sufficient, it is possible to obtain the elimination of a certain layer of the material, but the surface opened under the eliminated layer is not moulded to the depth necessary for closing the pores.
One more method of forming printing surfaces with the use of light radiation is to form on the surface of the porous thermo-plastic material a light-absorbing pattern of non-printing elements and to heat the surface by exposure to infra-red radiation, as in U.S. Pat. No. 4,064,205. The forming of the light absorbing pattern is accomplished by using a temporary cover sheet with a light absorbing coating and by transfer of the coating to selected areas of the printing surface by laser scanning. This method is used to form relief printing plates and is too complicated to use for the production of pre-inked stamps, as it requires the use of a powerful laser scanner.
Among the known technical solutions, the closest to the present invention is a method of manufacturing printing plates by heating the porous thermo-plastic material by light radiation (U.S. Pat. No. 3,742,853, B 41 D 7/00, Perkin-Elmer Corporation, 1973). The surface of porous thermo-plastic material is covered with a reflecting metal film. This film is eliminated by laser radiation in background areas. After the surface has been prepared in such a way, it is heated by intensive infra-red radiation, which makes the areas not screened by the reflective film melt and collapse. To accomplish this method, a laser is needed, powerful enough to evaporate the metal film on the surface when scanning it, and also a source of infra-red radiation. As the method is used for manufacturing of relief printing plates, simultaneously with heating, as a mechanical means, a pressure difference between the upper and lower surfaces of the porous material is applied, using for it, for example, a perforated plate connected to a vacuum system under the lower surface of the porous material. This method does not provide any additional means, applied simultaneously with the heating by light radiation, which would promote the closing of pores.